To facilitate an appreciation of the invention, this section may discuss the historical and technical background leading to the development of the invention, including observations, conclusions, and viewpoints that may be unique to an inventor. Accordingly, the background statements herein should not be construed as an admission regarding the content of the prior art.
Gamma-hydroxybutyric acid (GHB) is a putative neuromodulator that has profound physiological, pharmacological and biochemical effects in the brain and the peripheral system (Maitre, Prog. Neurobiol., Vol. 51, pp. 337-361 (1997)). The functions of γ-hydroxybutyric acid in the brain include anxiolytic effect, sleep modulation, anesthesia, and absence seizures. In addition, GHB regulates the neurotransmitter release of dopamine, opioids, glutamates and acetylcholine.
GHB has been used as an anesthetic agent for treating sleep disorder and alcohol dependence (see, e.g., Gonzalez, Psychopharmacology, Vol. 19, pp. 195-204 (2005), and Wong et al., Trends Pharmacol. Sci., Vol. 25, pp. 29-34 (2004)). GHB has been marketed as a bodybuilding and fat burning compound. Due to its euphoric effects, GHB has the potential for abuse (Bernasconi et al., TiPS, Vo. 20, pp. 135-141 (1999)). In 1990 GHB was banned for non-prescription sale, and in 2000 GHB was classified as a schedule I drug by the U.S. Food and Drug Administration.
GHB's mechanism of action remains to be established. Bernasconi et al. (1999) have suggested that the receptor for GHB is a GABAB receptor (Bernasconi et al., 1999). Others have merely postulated that GHB's receptor is a GPCR (Andriamampandry et al., The FASEB Journal, Vol. 17, pp. 1691-1693 (2003).
G-protein coupled receptors (GPCRs) are transmembrane receptor proteins that are responsible for the transduction of a diverse array of extracellular signals, including hormones, neurotransmitters, peptides, lipids, ions, light, odorants, nucleotides, fatty acid derivatives, and other chemical mediators (see, e.g., International Publication No. WO 02/00719). GPCRs are of particular importance to drug discovery because they have been established as excellent drug targets: they are the targets of 50% of marketed drugs. An increasing number of diseases have been found to be associated with GPCRs. Drugs targeting GPCRs have been used to treat a wide range of disorders from cardiovascular to gastro-intestinal to central nervous system (CNS) and others (Wilson et al., 1998, British J. of Pharmacology 125:1387-1392).
The GPCR-mediated signal transduction event is often initiated upon binding of a specific ligand to the GPCR. Each GPCR is composed of an extracellular N-terminal domain, seven distinct transmembrane segments, and an intracellular C-terminal domain. Binding of the ligand to the extracellular N-terminal domain of GPCR results in a conformational change that leads to activation of intracellular heterotrimeric GTP-binding proteins (G proteins) associated with the GPCR. These activated G proteins in turn mediate a variety of intracellular responses that regulate the cell physiology. Therefore, the ligand provides means of elucidating the physiological function of the GPCR as well as methods of screening for compounds that regulate the signal transduction activity of the GPCR.
Through sequence analyses, it was discovered that GPCRs belong to one of the largest superfamilies of the human genome: evaluated at over 1000 genes encoding GPCRs (see Civelli et al., 2001, Trends in Neurosciences 24:230-237; United States Patent Application Publication No. US 2003/0171541). A large number of putative GPCRs are described as orphan receptors because their natural ligands are unknown. Some of these uncharacterized orphan GPCRs may be useful as therapeutic targets. The identification of the specific ligand for a GPCR is the key to harnessing the potential benefits of these orphan GPCRs as potential therapeutics (Howard et al., 2001, Trends in Pharmacological Sciences 22:132-140) or drug discovery tools.
GPR81, also known as GPCR104, is an orphan G-protein coupled receptor (GPCR) (Lee et al., Gene, Vol. 275, pp. 83-91 (2001); see also US Patent Application Publication No. 2003/0171541). The receptor is up-regulated during T-cell activation (Mao et al., Genomics, Vol. 83(6), pp. 989-999 (2004)). GPR81 shares about 70% homology to nicotinic acid receptor HM74A and HM74; however, nicotinic acid does not activate GPR81 (Tunaru et al., Nat. Med., Vol. 9(3), pp. 352-355 (2003)). GPR81 is expressed in many tissues and highly expressed in fat and lymph nodes, and therefore this receptor is expected to play an important role in metabolism and immune functions. There is a desire to identify the endogenous ligand for GPR81, as this would facilitate the targeted development of pharmaceutical treatments for diseases and disorders mediated through modulation of this receptor's activity.